Leaves are the primary photosynthetic organs of plants, where transpirational water loss is regulated by stomata and where the greatest tensions are experienced within the Soil-Plant-Atmosphere continuum (SPAC). As water is transported in metastable state in plant xylem, it is prone to breakage that cause air embolism that block conduits, impair water transport and reduce plant performance. The water transport pathway through plants, and leaves in particular, is also intimately connected to the living-tissues outside the xylem that can also affect water transport. We combined high-resolution x-rays micro-Computed Tomography ([mu]CT) and comprehensive 3D analysis with hydraulic and other physiological measurements to study how the leaf hydraulic system in wine grape varieties and species from a diverse set of ecological backgrounds respond to progressive dehydration. We provide novel understanding of the anatomical and structural changes in leaves and its impacts on leaf hydraulics and plant performance during progressive dehydration. The integration of the entire leaf (i.e. petiole and lamina) as well as the linkage between the responses of different varieties of crop species such as grapevines and an ecological approach using a set of species from diverse ecological backgrounds sheds light on the structural drivers of leaf hydraulic decline during progressive drought. In the first chapter, we present an overview of this dissertation and how it fits in the field of plant ecophysiology. In the second chapter, we studied petioles and leaf laminas of a set of five species from diverse ecological backgrounds and combined with leaf hydraulics to understand the general drivers of leaf xylem hydraulic decline during progressive drought. We found across species that the largest diameter xylem conduits in petioles and midribs are the first to embolize and only at severe dehydration. We also found that conduits in leaf minor veins are extremely resistant to cavitation and rarely embolized. In the third chapter, we present our findings of the drivers of leaf hydraulic conductance (K[subscript leaf]) and stomatal conductance (g[subscript s]) decline in two of the most widely cultivated wine grape varieties. By combining hydraulic and physiological measurements with scans of intact leaves across a range of water potentials, we found that leaf xylem embolism is not the cause for K[subscript leaf] and g[subscript s] decline during mild dehydration. Lastly, in the fourth chapter we studied anatomical responses to progressive dehydration in intact petioles of two common wine grape varieties known to exhibit differences in leaf drop under drought and its implications for water relations. We report for the first time the phenomenon of petiole pinching, a mechanical damage to grapevine petioles at severe water stress caused by collapsible pith that may block water flow and be related to leaf drop during drought. Petiole shrinkage during dehydration was caused by water loss in the collapsible pith cells, and this tissue was rapidly recharged upon rehydration. The collapse of the pith cells was also associated with collapse of water-filled xylem conduits in intact grapevine petioles that was reversible after relaxation of xylem tensions upon rehydration that was also associated with xylem conduit refilling in vessels that had embolized.